Coating agents obtained by hydrolyzing or partially hydrolyzing hydrolyzable organosilanes, or coating agents in which colloidal silica is mixed with such coating compositions have conventionally been known as coating agents for forming water-resistant coating films intended for the surface protection of steels like stainless steel, non-ferrous metals like aluminum, inorganic building materials such as concretes or slates, woods, plastic substrates, and the like.
For example, Japanese Patent Laid-Open Publications SHO 51-2736, SHO 51-2737, SHO 53-130732, and SHO 63-168470 have proposed coating agents composed of organoalkoxysilanes, hydrolyzates and/or partial condensates of the organoalkoxysilanes, and colloidal silica, in which alkoxy groups are converted to silanols by excess water. However, while coating films obtained from these coating agents are excellent for the use of substrate protection by virtue of their high hardness and good weatherability, they are poor in toughness and, in the case of coating films having a film thickness of 10 .mu.m or more, would easily develop cracks during heat curing process, in outdoor use, or at the eruption of abrupt temperature changes. Also, in the coating process, it is difficult to control the thickness of a coating film strictly below 10 .mu.m or less, such that the film thickness would be more likely to exceed 10 .mu.m on the coating surface or around the coating article or at recessed portions of the coating surface, thus likely causing production faults. In order to obtain desired coating film characteristics, the coating films obtained from these coating agents need to be subjected to a heat treatment at a high temperature of about 100.degree. C. or more or for a long time, such that the coating agents could not be used in some cases depending on the molding method or size of the substrate, or weatherability, or outdoor and other locations, inconveniently. Moreover, there has been an issue that these coating resin compositions are high in reactivity of silanol resulting from hydrolysis of alkoxysilane, so that their condensation reaction gradually proceeds even at room temperature, causing gelation to occur, with the result of poor stability. In particular, when it is attempted to derive a paint from these coating compositions as a vehicle of paint by adding pigments, the stability would be further deteriorated such that the coating resin compositions could not be formed into a paint, as a drawback.
As disclosed in Japanese Patent Laid-Open Publication SHO 64-168, a coating agent has also been proposed in which alkoxy groups are converted into silanols by adding water as a curing agent and a catalyst to partial hydrolyzates or partial condensates of alkoxysilane immediately before the coating process. This coating agent, however, is also so poor in toughness that cracks would easily develop with the film thickness of 10 .mu.m or more. While this coating agent is stable in preservation and keeps relatively stable even if formed into a paint with pigments added, it is necessary for obtaining desired coating film characteristics to subject the coating agent to a heat treatment at a high temperature of about 100.degree. C. or more or for a long time. Thus, the coating agent could not be used depending on the molding method or size of the substrate, or weatherability, or outdoor and other locations.
For the purpose of solving these and other drawbacks, a coating agent which is composed of a prepolymer containing a silicon alkoxide as a main component, a curing catalyst, and water and which cures at around room temperature has been proposed in Japanese Patent Laid-Open Publication SHO 63-268772. However, this coating agent has not been improved in toughness, so that it is impossible to do processings of precoating metals after coating, processings of polycarbonate plates after a coat treatment, and the like. Also the coating agent is poor in coatability and curability, showing a drawback that the curability of the coating agent tends to be affected by humidity.
A coating agent which contains a partial hydrolyzed oligomer of organosilane, a silanol-group-containing polyorganosiloxane, and a curing catalyst, has been disclosed in Japanese Patent Laid-Open Publication HEI 4-175388. However, while this coating agent has a merit that the toughness and curability of the coating agent are improved and are not affected by humidity, the toughness of the coating agent has not been improved sufficiently.
In summary, coating agents containing organoalkoxysilane or a hydrolyzate of organoalkoxysilane as a main component have proved to be able to produce coating films that are high in hardness, unlikely to be flawed, and good at weatherability. However, because of poor toughness, the coating films will be easily cracked in the coating process or during use, which can be seen noticeably especially when the film thickness becomes 10 .mu.m or more. Further, the coating agents need a baking process at a high temperature of 100.degree. C. or more, so that they indeed can be applied to coating at factories, but have difficulty in outdoor or field coating. Besides, the coating agents are defective in high reactivity of their coating fluid and therefore poor in storability of their coating formulations.
Resins having, as the main chain, acryl, polyester, epoxy, polyether, vinyl, or other organic resins as well as functional groups of hydrolyzable silane have also been reported. They can be seen, for example, in Japanese Patent Laid-Open Publications HEI 5-287206 and HEI 5-302007. However, since the main chain is composed of organic resins, those resins are inferior in weatherability and hardness to the resins having organoalkoxysilanes as the main component. Otherwise, reactive resins having linear polysiloxanes in their main chains and functional groups of polymerizable acrylate in their terminals or side chains have also been proposed in Japanese Patent Laid-Open Publications HEI 5-72928 and HEI 5-178998. However, since the main chain is a linear polysiloxane, these resins cannot attain sufficient hardness, and in some cases, they are of rubber elastic material, thus unsuitable for coating-use resins.
Aforementioned concretes, cement substrates, or inorganic curing bodies are indeed excellent materials good at heat resistance and durability, but defective in that moisture would invade when their surfaces are not coated, and that they are inferior in stain resistance and acid resistance. Besides, uncoated appearance of those materials are not of a beauty in many cases.
To make up for these defects, it has been practiced to coat their surface with organic coating materials. However, organic coating materials are poor in weatherability and low in film hardness so that they are easily flawed, as a drawback.
For this reason, attempts have been made to apply inorganic coating agents related to water glass, instead of the organic coating materials. However, these attempts have shown no satisfactory results in terms of the generation of efflorescence and porosity.